1. Field of the Invention
This invention relates to an image forming apparatus having electron emission elements and spacers in a vacuum envelope.
2. Description of the Related Art
Flat panel displays of large areas have been the focus of much research and development in recent years.
In general, an image forming apparatus using electrons is equipped with an envelope for maintaining a vacuum, electron sources and their drive circuitry for emitting electrons, an image forming member having phosphors or the like for emitting light owing to electron bombardment, accelerating electrodes for accelerating electrons toward the image forming member, and a high-voltage power supply for the accelerating electrodes. Further, in an image forming apparatus using a flat envelope as in the manner of a flat-panel display having a large screen area, there are cases where supporting columns (spacers) are disposed within the envelope as structures resistant to atmospheric pressure.
Two types of elements, namely thermionic cathode elements and cold cathode elements, are known as electron emission elements for constructing the electron sources mentioned above. Examples of cold cathode elements are surface-conduction electron emission elements, electron emission elements of the field emission type (abbreviated to "FE" below) and metal/insulator/metal type (abbreviated to "MIM" below).
An example of the surface-conduction electron emission element is described by M. I. Elinson, Radio. Eng. Electron Phys., 10 (1965). There other examples as well, as will be described later.
The surface-conduction electron emission element makes use of a phenomenon in which an electron emission is produced in a small-area thin film, which has been formed on a substrate, bypassing a current parallel to the film surface. Various examples of this surface-conduction electron emission element have been reported. One relies upon a thin film of SnO.sub.2 according to Elinson, mentioned above. Other examples use a thin film of Au [G. Dittmer: "Thin Solid Films", 9.317 (1972)]; a thin film of In.sub.2 O.sub.3 /SnO.sub.2 (M. Hartwell and C. G. Fonstad: "IEEE Trans. E.D. Conf.", 519 (1975); and a thin film of carbon (Hisashi Araki, et al: "Vacuum", Vol. 26, No. 1, p. 22 (1983).
FIG. 45 is a plan view of the element according to M. Hartwell, et al., described above. This element construction is typical of these surface-conduction electron emission elements. As shown in FIG. 45, numeral 3001 denotes a substrate. Numeral 3004 denotes an electrically conductive thin film comprising a metal oxide formed by sputtering and is formed into a flat shape resembling the letter "H" in the manner illustrated. The conductive film 3004 is subjected to an electrification process referred to as "electrification forming", described below, whereby an electron emission portion 3005 is formed. The spacing L in FIG. 45 is set to 0.5.about.1 mm, and the spacing W is set to 0.1 mm. For the sake of illustrative convenience, the electron emission portion 3005 is shown to have a rectangular shape at the center of the conductive film 3004. However, this is merely a schematic view and the actual position and shape of the electron emission portion may be represented in other ways.
In the above-mentioned conventional surface-conduction electron emission elements, especially the element according to Hartwell, et al., generally the electron emission portion 3005 is formed on the conductive thin film 3004 by the so-called "electrification forming" process before electron emission is performed. According to the forming process, a constant DC voltage or a DC voltage which rises at a very slow rate on the order of 1 V/min is impressed across the conductive thin film 3004 to pass a current through the film, thereby locally destroying, deforming or changing the property of the conductive thin film 3004 and forming the electron emission portion 3005, the electrical resistance of which is very high. A fissure is produced in part of the conductive thin film 3004 that has been locally destroyed, deformed or changed in property. Electrons are emitted from the vicinity of the fissure if a suitable voltage is applied to the conductive thin film 3004 after electrification forming.
Known examples of the FE type are described in W. P. Dyke and W. W. Dolan, "Field Emission", Advance in Electron Physics, 8.89 (1956), and in C. A. Spindt, "Physical Properties of Thin-Film Field Emission Cathodes with Molybdenum Cones", J. Appl. Phys., 47, 5248 (1976).
A typical example of the construction of an FE-type element is shown in FIG. 46, which is a sectional view of the element according to Spindt, et al., described above. The element includes a substrate 3010, emitter wiring 3011 comprising an electrically conductive material, an emitter cone 3012, an insulating layer 3013 and a gate electrode 3014. The element is caused to produce a field emission from the tip of the emitter cone 3012 by applying an appropriate voltage across the emitter cone 3012 and gate electrode 3014.
In another example of the construction of an FE-type element, the stacked structure of the kind shown in FIG. 46 is not used. Rather, the emitter and gate electrode are arranged on the substrate in a state substantially parallel to the plane of the substrate.
A known example of the MIM type is described by C. A. Mead, "Operation of Tunnel Emission Devices", J. Appl. Phys., 32, 646 (1961). FIG. 47 is a sectional view illustrating a typical example of the construction of the MIM-type element. The element includes a substrate 3020, a lower electrode 3021 consisting of a metal, a thin insulating layer 3022 having a thickness on the order of 100 .ANG., and an upper electrode 3023 consisting of a metal and having a thickness on the order of 80.about.300 .ANG.. The element is caused to produce an emission from the surface of the upper electrode 3023 by applying an appropriate voltage across the upper electrode 3023 and lower electrode 3021.
Since the above-mentioned cold cathode element makes it possible to obtain an electron emission element at a lower temperature in comparison with a thermionic cathode element, a heater for applying heat is unnecessary. Accordingly, the structure is more simple than that of the thermionic cathode element, and it is possible to fabricate elements that are more slender. Further, even though a large number of elements are arranged on a substrate at a high density, problems such as fusing of the substrate do not readily arise. In addition, the cold cathode element differs from the thermionic cathode element in that the latter has a slow response speed because it is operated by heat produced by a heater. Thus, an advantage of the cold cathode element is a quicker response speed.
For these reasons, extensive research into applications for cold cathode elements is being carried out.
By way of example, among the various cold cathode elements, the surface-conduction electron emission element is particularly simple in structure and easy to manufacture and therefore is advantageous in that a large number of elements can be formed over a large area. Accordingly, research has been directed to a method of arraying and driving a large number of elements, as disclosed in Japanese Patent Application Laid-Open No. 64-31332, filed by the applicant.
Further, applications of surface-conduction electron emission elements that have been researched are image forming devices such as image display devices and image recording devices, as well as charged beam sources, etc.
As for applications to image display devices, research has been conducted with regard to such devices using, in combination, surface-conduction type electron emission elements and phosphors which emit light in response to irradiation with an electron beam, as disclosed, for example, in the specifications of U.S. Pat. No. 5,066,833 and Japanese Patent Application Laid-Open (KOKAI) Nos. 2-257551 and 4-28137 filed by the present applicant. The image display device using the combination of the surface-conduction type electron emission elements and phosphors is expected to have characteristics superior to those of the conventional image display device of other types. For example, in comparison with a liquid-crystal display device that has become so popular in recent years, the above-mentioned image display device emits its own light and therefore does not require back-lighting. It also has a wider viewing angle.
A method of driving a number of FE-type elements in a row is disclosed, for example, in the specification of U.S. Pat. No. 4,904,895 filed by the present applicant. A flat panel-type display apparatus reported by Meyer et al., for example, is known as an example of an application of an FE-type element to an image display apparatus. [R. Meyer: "Recent Development on Microtips Display at LETI", Tech. Digest of 4th Int. Vacuum Microelectronics Conf., Nagahara, pp. 6.about.9, (1991).]
An example in which a number of MIM-type elements are arrayed in a row and applied to an image display device is disclosed in the specification of Japanese Patent Application Laid-Open Nos. 3-55738 filed by the present applicant.
The inventors have experimented with cold cathode elements consisting of various materials manufactured by various methods and having a variety of structures. Furthermore, the inventors have investigated multiple electron beam sources, consisting of an array of a number of cold cathode elements, and image display devices which employ these multiple electron beam sources.
In a flat panel image display device, an electron emission element, an image forming member and various electrodes are placed within a vacuum envelope. The various electrodes include wiring electrodes for supplying the electron emission element with current, an accelerating electrode for applying a high voltage to the image forming member, and electrodes (a focusing electrode, modulating electrode, deflecting electrode, etc.) for controlling the electron beam. Of course, all of these electrodes are not necessarily required, and it will suffice if only the electrodes for controlling the electron beam are provided as necessary.
In such a flat panel image display device, mechanical strength capable of withstanding atmospheric pressure cannot readily be achieved by a vacuum envelope alone. For this reason, the general practice is to provide supporting columns (spacers) inside the vacuum envelope. However, a flat panel image display device of this kind has the following problems:
Specifically, the inventors have discovered cases where the light emitting position of the phosphor constituting the image forming member (the position bombarded by electrons) and the shape of the emitted light deviate from their design values.
In particular, when an image forming member for a color image is used, there are cases where a decline in brightness and the occurrence of a color shift arise along with a shift in the light emitting position. It has been confirmed that this phenomenon occurs in the vicinity of the supporting column (spacer) disposed between the electron source and the image forming member.
Further, the inventors have found that the prime cause of the aforementioned phenomenon is the electrons emitted from the electron source.
In the above-described image forming apparatus, the electrons emitted from the electron source bombard the phosphors constituting the image forming member as well as any gas remaining in the vacuum, although the probability of this occurring is low. It has been found that some scattered particles (ions, secondary electrons, neutral particles, etc.) produced at a certain probability at the time of these bombardments impact upon the exposed portion of the support column (spacer) within the image forming apparatus, thereby charging the exposed portion. Owing to the electric charge, the electric field changes in the vicinity of the exposed portion, thereby causing a shift in the path of the electrons. It is believed that this causes a change in the light emitting position of the phosphors and a change in the shape of the light emission.
Further, the inventors have found that mainly a positive charge accumulates at the exposed portion based upon the change in the light emitting position and the change in the shape of the light emission. The cause is believed to be either charging due to the accumulation of positive ions included among the scattered particles or the occurrence of positive charging owing to the emission of secondary electrons generated when the scattered particles bombard the exposed portion mentioned above.
In order to solve these problems, a method of preventing charging by covering the surface of the spacer with a resistive film is already known. According to this method, the ability to prevent charging is improved by reducing the electrical resistance of the resistive film. However, when the electrical resistance is made small, the current which flows in a steady state increases, thereby raising power consumption.
In a case where the surface of the spacer cannot be covered with the resistive film evenly, the current which flows through the resistive film develops a non-uniform distribution, resulting in an undesirable electric potential distribution being produced on the spacer surface. If this problem arises, the path of the electron beam is affected. Consequently, a shift in the light emitting position occurs, though the shift is not at serious as that caused when the spacer is not covered with the resistive film.
Accordingly, there has been reported a device in which the surface of a spacer is covered with a resistive film and a portion of the spacer surface is provided with an electrode in an attempt to solve the problems of power consumption and deviation of the light emitting position. Specifically, the specification of U.S. Pat. No. 5,532,548 discloses an arrangement in which electrodes parallel to a face plate and a back plate are formed on part of the surface of a spacer and a desired potential distribution is obtained by controlling the voltage applied to the electrodes. FIGS. 40, 41 and 42 are diagrams disclosed in the prior-art U.S. Pat. No. 5,532,548. A flat panel display 5010 includes a face plate 5012, a back plate 5014 and a side wall 5016. These components form a hermetic envelope 5018. The interior of the envelope 5018 has a light-emitting area of length L1 obtained by coating the inner side of the face plate 5012 with phosphors. One or more spacers 5020 supports the face plate 5012 so as to oppose the back plate 5014. Each spacer 5020 has a length L and electrodes 5028 formed on each spacer 5020 each have a length of L2. If the spacers 5020 are formed from an insulator, the side wall of the spacer is provided with a coating 5026 of a resistor or is surface-doped.
According to U.S. Pat. No. 5,532,548, electrodes having a length (L2) at least greater than that L1 of the light-emitting area are formed on the spacers (of length L), which are longer than the light-emitting area (of length L1) that forms the image. It should be noted that a disclosure similar to that of U.S. Pat. No. 5,532,548 is made in U.S. Pat. No. 5,614,781 as well.
A problem with this device is that the electrodes 5028 formed on the spacer surface tend to produce a spark discharge. If a spark discharge is produced, the phosphors and electron emission elements may sustain damage from which recovery is not possible. Accordingly, in a device of this kind, the voltage impressed upon the phosphors must be suppressed in such a manner that spark discharge will not occur. A practical problem which arises is that a displayed image having a high brightness cannot be obtained.
As a result of continuing their investigations, the inventors have found that the locations at which spark discharge occurs are the points indicated by the arrows Bd in FIG. 43. More specifically, these locations are on the boundary between the electrode 5028 and a spacer, and reside at the corners of the electrode 5028.
Further, the inventors have studied spacers in which the length L of the spacer and the length L2 of the electrodes are equal, as illustrated in FIG. 44. This, however, failed to solve the problem of spark discharge. In other words, it was found that spark discharge occurs at the points indicated by the arrows Bd.